This invention relates to multilayer electric cables and materials therefor.
Multilayer electric cable constructions are well known in the art and are utilized for the transmission of medium and high voltage electric current. The basic single conductor medium voltage cable commonly utilized by the industry today comprises a conductor surrounded by an extruded strand shield (ESS). Superimposed over the extruded strand shield is an insulation layer that, in turn, has strippably bonded thereto an extruded insulation shield (EIS). A metallic shield comprising flat copper tapes or round wires are helically positioned over the extruded insulation shield to complete the shielding system for the cable. An outer jacket can then be placed, if necessary, over the wire or tape to provide the final, finished cable construction. A particularly preferred multilayer cable utilizes corrugated wires embedded in the extruded insulation shield and is illustrated in U.S. Pat. No. 3,474,189, the teachings of which are incorporated by reference herein. This type of cable is manufactured and sold by the Anaconda-Ericsson Inc. under the trademark "UniShield".
It would be desirable to manufacture multilayer electric cable in a continuous process to minimize handling of the cable during the intermediate stages of its production and to minimize production time and in-process storage. It would also be desirable to utilize dry conditions (a high temperature dry gas atmosphere) to cure the various polymeric materials in the cable. Dry curing avoids the moisture infiltration problems and drying steps necessary in the prior art processes.
It has been discovered, however, that the conventional extruded insulation shield compositions are not adequately suited for high temperature dry curing. Thermoplastic compositions applied and then cured simultaneously with the insulation layer will completely bond together and cannot be stripped apart during subsequent splicing operations. Existing strippable, thermosetting compositions are either not sufficiently flame retardant, deformation resistant or thermally stable to withstand the high temperature present in a dry cure process, e.g. 200.degree. C. or higher.
Representative prior art pressurized, high temperature curing processes for continually vulcanizing and manufacturing electric cables are illustrated in U.S. Pat. Nos. 3,645,656; 3,846,528 and 3,901,633, the teachings of which are incorporated by reference herein.